1. Field of the Invention
The invention is directed to the field of surge arresters used to attempt to protect high voltage systems from the effects of overvoltage incidents created by lightning strikes and more particularly to the construction of such surge arresters to attempt to prevent injury to personnel or equipment due to the catastrophic failure of such surge arresters during overvoltage incidents.
2. Description of the Prior Art
In surge arresters of the prior art, MOV blocks are stacked together and capped at each end with a fitting including a threaded stud. The block and cap stack is then placed in an elastomeric housing through an open end with one of the studs projecting from the housing closed end for connection to a support or connector probe. A metal cap is fixed over the open end of the housing with the second stud projecting therethrough for connection to a ground conductor. Although the elastomeric housing supporting the cap is locally weakened to encourage the downward movement of fragments of the MOV blocks and fittings should these elements fracture due to catastrophic failure of the arrester, the elastomeric housing possesses insufficient strength to prevent the scattering of fragments, contain them and force them downwardly to minimize injury to persons in proximity to the arrester or nearby equipment.
In U.S. Pat. No. 4,404,614 issued Sep. 13, 1983, a surge arrester made up of a number of blocks 30 is placed in an EPDM rubber housing 12. To add structural integrity to the arrester "an inner tubular liner 36 disposed concentrically within and extending the entire length of chamber 14 between the internal components of the latter and the inner surface of the housing 12. This liner is constructed of a material having a high bursting strength, preferably resin-impregnated fiberglass (specifically epoxy resin-impregnated filament-wound fiberglass). An intermediate sleeve 38 is disposed concentrically between and extends the entire length of liner 36 and the inner surface of housing 12. This sleeve is constructed of a moisture-impervious material preferably glass flakes in an epoxy matrix."
Such an arrangement provides added strength but fails to provide an air-free non-electrically ionizable environment about the arrester blocks to minimize internal electrical arcing which can lead to block destruction during overvoltage incidents.
The patent to Thuillier, U.S. Pat. No. 4,864,456 issued Sep. 5, 1989, shows a lightning arrester which uses a filament winding to provide radial binding without significant axial compression. "The filament winding thus has the sole function of holding the pellets together . . . " It also adheres to the pellets and the spacers but because of the undulating surface of pellets and spacers, the air is not eliminated within housing 5 or between the winding 4 and the pellets 1 and spacers 2.
To provide a surge arrester having excellent heat transfer properties and improved tensile and cantilever strengths, U.S. Pat. No. 4,656,555 issued Apr. 7, 1987, uses a wrap of filament windings 14 over MOV blocks 11, 12 and terminal pieces 16, 18 before insertion in a weathershed housing 20. The arrangement of windings 14 and blocks 11, 12 are not sealed against the intrusion of air therebetween.
Bergh et al., U.S. Pat. No. 4,467,387 issued Aug. 21, 1984, shows a wound tube 22 of glass fibers in bonding resin but spaced from blocks 42 by elastomeric sleeves 42a and resilient balls 44. Air can be entrapped adjacent the blocks 42 inside the insulative tube 22. U.S. Pat. No. 4,851,955 issued July 25, 1989, to Doone et al. shows MOV blocks 2 in a glass-reinforced plastic shell 5 within heat-shrink sleeve 6. Shell 5 is bonded to the outer surfaces of MOV blocks 2, heat sink/spacer blocks 3 and terminal blocks 4.